PhilW

I need help to do a rough calculation of the loss in acceleration in switching from a 255/40/17 to a 275/40/17 tire. I would use the same wheel, assume it weighs 22 pounds and the car weighs 2760 pounds.

I think the only meaningful variables are

1) the increased radius at which the force is applied. (0.25 inches)
2) the increased weight of the tire. (2.2 pounds )
3) the weight of the tire is now further out from the center of the wheel. ( less than 0.25 inches)
4) each rotation of the larger tire moves the car 1.93 inches further down the road.

Any takers?

Thanks

Phil W.

GhettoRacer

[quote]Originally posted by PHILW:
<strong>I need help to do a rough calculation of the loss in acceleration in switching from a 255/40/17 to a 275/40/17 tire. I would use the same wheel, assume it weighs 22 pounds and the car weighs 2760 pounds.

I think the only meaningful variables are

1) the increased radius at which the force is applied. (0.25 inches)
2) the increased weight of the tire. (2.2 pounds )
3) the weight of the tire is now further out from the center of the wheel. ( less than 0.25 inches)
4) each rotation of the larger tire moves the car 1.93 inches further down the road.

Any takers?

Thanks

Phil W.</strong><hr></blockquote>

I suck at math. But, wouldn't it be easier to just datalog the difference?

PhilW

Yeh, but its winter and I won't get into my car until April/May and I need something to think about.

Phil W.

Bill Verburg

The increased weight will have a negligable influence on acceleration so only the affect of gearing need be considered. Without knowing the absolute size of the tire not enough info has been given. Easiest and most accurate way to calc is to use the manufacturers revs per mile which should be measured from the actual physical dimensions of the tire. The calc then goes as follows;

rpms: revs per mile of the short tire
rpmt: revs per mile of the tall tire

rpms/rpmt X 100% - 100% = % taller gearing
ex.- For a 25.1" tall 255/40x17 compared to a 25.7" tall 275/40x17
809/798 = 1.01378 x100% =101.378% - 100% = 1.378% taller gearing

Russ Murphy

[quote] Any takers? <hr></blockquote>
All right, I'll give it a shot. What's your axle ratio and what is your engine torque?

richard glickel

I like fig newtons. And, I really like apple newtons.

I'm not a Wayne Newton fan; though I liked when Matthew Broderick lipped-synched "Dankeshein" in Ferris Bueller's Day Off.

Richard
'87 Carrera-3.6L

PhilW

Thanks Russ, the car is a 93 911 RSA/C2. The final drive is 3.4444 (source Bentley Publishers) The peak torque is 247 lb-ft at the engine, but lets assume an average of 235 lb-ft (source B&B Fabrication).

Thanks again

Phil W.

Russ Murphy

Allright, I'm sure someone will check my work (which would be a good thing) however here's what I've come up with (with the help of my chassis engineering book).

The difference in acceleration between the two sets is .008 g less on the bigger set. this assumes 2750 lbs for the car, 3.44 for the gearing and that the additional weight is in the chassis NOT rotating weight. So let's assume a long straight where you would be accelerating for 8 seconds. Your change in velocity from the turn exit till the end of 8 seconds would be 49.35 mph for the original set and 47.95 mph for the new set. In the text it is stated that the speed change for additional weight that rotates @ axle speed will be three times greater than that in the chassis, so I'll say you'll be going 4.2 mph slower at the end of a 8 second straight with the new set. <img src="graemlins/icon107.gif" border="0" alt="[icon107]" />

Stem

UH... yeah.....My engine has 6 cylinders!!!

Russ Murphy

[quote] UH... yeah.....My engine has 6 cylinders!!! <hr></blockquote>
Come again??? I need a translation.

RSAErick

[quote]Originally posted by Russ Murphy:
<strong>.....so I'll say you'll be going 4.2 mph slower at the end of a 8 second straight with the new set. <img src="graemlins/icon107.gif" border="0" alt="[icon107]" /> </strong><hr></blockquote>

Wow! This difference is enormous! I'm no physics buff, so I wouldn't dare to challenge your findings. But I do have some questions:

At what point does a smaller diameter wheel actually become a hindrance or at what point does a larger wheel become an advantage. I'm assuming that a 1" diameter wheel would accelerate like mad, but only up to the point at which the gearing of the car resulted in a ridiculously slow "top" speed (obviously not ideal for road track use - but maybe good for "short" courses like autocrosses).

To simplify what I am getting at, is it possible to calculate at what length straight-away (in seconds of acceleration?) the larger wheel becomes an advantage.

I am fascinated by these calculations and their substantial differences. Thanks for performing them!

Erick

Russ Murphy

[quote] Wow! This difference is enormous! <hr></blockquote>
Well, keep in mind an 8 second straight is probably pretty long relatively speaking. And I'm making no guarantees about these numbers being absolutely correct, just my best effort. They do compare respectively and proportionally to the examples in the text.
Based on the formula, I don't see any way that a larger wheel can help you accelerate.

RSAErick

[quote]Originally posted by Russ Murphy:
<strong>
Based on the formula, I don't see any way that a larger wheel can help you accelerate.</strong><hr></blockquote>

Well, given the fact that there are 5 gears (say these are fixed), there must be a point at which the acceleration advantages of a smaller diameter wheels are outweighed by the limitations in velocity. Likewise, there must be a point at which the potential velocity of a larger wheel is unrealized on a particular straight-away because of the limitations in acceleration to get to that velocity.

Regardless, I was very surprised by the significant difference in velocity between two 17" wheels that differ in diameter only because of the tire dimensions. Imagine going with a 16" or 15" wheel. And for all those people who "upgrade" to 18" or 19" wheels, it is interesting to note what kind of difference that change may have.

Erick

PhilW

Thanks Russ, this is good stuff -

The butt dyno was telling me that with the larger circumferance tire my car had notiably less get up and go from 60 to 140 than with the smaller. I don't trust my personal dynos yet since I've been at this all of 14 months. I do wish I could get a 275/35 17 track tire.

I have yet to run out of engine on a track, I usaly lift around 140. <img src="graemlins/crying.gif" border="0" alt="[crying]" /> This brings me to part two of my question. What is the rough increase in latteral & braking grip on a 255/40 vs. a 275/40 tire, same assumptions as above.

Maybe I should have pursued that 3/2 program with Washington U.

Thanks again

Phil W.

Russ Murphy

Erick,
So you're saying that what you lose in accelerative capacity by going to the bigger gear you can gain back by choosing a smaller gear? Careful here, I'm wading into the deep end of the pool but I think that's correct.

F= (engine torque) (gear ratio) / R tire radius